Work vehicles, such as tractors and other agricultural vehicles, having continuously variable transmissions (CVTs) are well known. CVTs generally allow for enhanced performance by providing continuous and smooth gear ratio changes. Typically, CVTs have a hydro-mechanical configuration such that power from the engine flows in parallel through both a hydrostatic power unit and a planetary power unit of the CVT. During operation, fluid displacement of the hydrostatic power unit is varied to continuously change the output to input ratio of the transmission (i.e., the transmission ratio). This is accomplished by adjusting the angle of a swash plate of a variable displacement fluid pump and/or motor of the hydrostatic power unit.
CVTs are often configured with the capability to perform shuttle shifts, wherein the direction of travel of the work vehicle may be reversed without requiring the operator to change gears or use the clutch. However, the hydrostatic power units utilized within many CVTs are configured such that, at a zero ground speed for the work vehicle, the swash plate is at full displacement or near full displacement, in one direction or the other, depending on the range selected, direction of travel and possibly other factors. Thus, when shuttle shifting, the work vehicle is often required to come to a complete stop as the swash plate is moved across its full range of travel in order to allow the direction of the work vehicle to be reversed. Such stopping of the work vehicle is undesirable in most instances.
To avoid stopping the work vehicle, control strategies have also been developed that provide for the on-coming directional clutch to be slipped while the swash plate angle is being adjusted. However, such a control strategy reduces the efficiency of the work vehicle due to the amount of energy dissipated while the clutch is being slipped. In addition, the thermal cycling caused by clutch slipping can result in significant damage to the directional clutches.
Accordingly, a system and method that provides for a CVT of a work vehicle to be operated within a pure hydrostatic mode to allow for seamless shuttle shifting and high efficiency low speed operation would be welcomed in the art. In addition, a system and method for controlling the operation of the CVT as it is transitioned between the hydrostatic mode and its normal hydro-mechanical mode would also be welcomed in the art.